A method for producing a nitride semiconductor device. The method comprises providing a substrate made of a material other than a nitride semiconductor. The material has a hexagonal crystal structure. An upper face of the substrate has at least one flat section. The method further comprises growing a first nitride semiconductor layer on the upper face of the substrate. The first nitride semiconductor layer is made of monocrystalline AlN. The first nitride semiconductor layer has an upper face that is a +c plane. The first nitride semiconductor layer has a thickness in a range of 10 nm to 100 nm. The method further comprises growing a second nitride semiconductor layer on the upper face of the first nitride semiconductor layer. The second nitride semiconductor layer is made of InXAlYGa1-X-YN (0≦X, 0≦Y, X+Y